Illuminating the pathological, immunohistochemical, and molecular investigations of myelocytomatosis in chicken embryos.

Avian leukosis virus Subgroup-J (ALV-J) is an avian retrovirus affecting a wide range of avian species, and has posed a great threat to the local poultry with significant losses. In Egypt, ALV-J has been actively monitored in various breeder flocks. This study investigated 200 eggs from ALV-J positive breeders, confirmed positive-polymerase chain reaction (PCR)-ALV-J, collected from a hatchery between January 2022 and December 2023. 200 tissue specimens (liver, heart, spleen, kidney, and lung) were collected from suspected embryos for PCR test, immunopathological identification, and sequencing analysis. To the best of our understanding, this is the first study specifically detecting ALV-J infection in chicken embryos in Egypt. The current study focuses on the genetic evolution and sequencing analysis of ALV-J isolates from chicken embryos across three Egyptian governorates: El-Sharkia, Al-Qalyubiya, and New Valley. Postmortem findings of the infected embryos showed stunting, curling, dwarfing, hemorrhagic body surface, enlarged liver, and congestion of chorio-allantoic membranes (CAM). Histopathological observation demonstrated no lymphoid or myeloid cell infiltrations, only degenerative changes and congestion of the examined organs existed. The immunohistochemical staining confirmed that the viral-positive signals were visceral tissues as the spleen, liver, and kidney. Only 30 samples were PCR-positive for ALV-J gp85 gene at size of 545 base pairs with a prevalence rate of 15%. Two ALV-J positive samples were sequenced and deposited in the Genbank under accession numbers (PQ119499 - PQ119500, ALV-J-II). ALV-J-gp85 gene phylogeny showed that the AlQalyubiya-1-EGYALVJ-env isolate is highly genetically correlated to Chinese strains (KJ179914-ALV-J-GD27, KJ179912-ALV-J-GD19, KJ179911-ALV-J-GD31) with nucleotide and amino acid identities of 98-100%; respectively. Moreover, Newvalley-2-EGYALVJ-env isolate is close similar to previous mentioned Chinese strains with nucleotide identity percentages of 98% and amino acid identity percentage 97%, 96%, 96%; respectively. This study confirmed that our two ALV-J isolates are not completely similar to the current Egyptian isolates. Also, ALV-J infection was continuously distributed in the breeders and considered one of the factors contributing to the tumor epidemics.